Microelectromechanical systems (MEMS) is the technology of very small devices, that merge at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are made up of components between 1 to 100 micrometers in size (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometers (20 millionths of a meter) to a few millimeters (i.e. 0.02 to >1.0 mm). MEMS usually consist of a central unit that processes data (the microprocessor) and several components that interact with the surroundings such as microsensors. At these sizes, the standard constructs of classical physics are not always useful. Because of the large surface area to volume ratio of MEMS, surface effects such as electrostatics and wetting dominate over volume effects such as inertia or thermal mass.
MEMS are finding more and more applications in commercial, industrial, military and medical markets. Today microelectromechanical systems are used to drive such things as: i) micro-mirror arrays for optical switching applications; ii) piezo-electric transducers for ultrasound; iii) cholesteric molecules for sensing and no-power display applications; iv) gyroscopes; v) accelerometers; vi) radar and other switches; vii) microfluidic pumps; viii) camera and eye lenses; ix) biological microarrays; x) biological cells based sensors and transducers such as heart cell actuators, and various other mechanical assemblies.
While the applications for MEMS are as varied as the imagination, many MEMS applications share a common requirement for a high voltage interface IC or integrated circuit capable of driving and/or interfacing and/or generating high voltage precision waveforms with extreme accuracy, over large numbers of channels, in an area not so large as to eliminate the benefits of the microelectromechanical device whose primary benefit is usually to save space and power so as to be incorporated into portable or miniature products. They also are often deployed in applications which operate from batteries or for which power must be minimized and often the small size of the MEMs sensors produces a desire for small form factors for the interfaces, biasing circuits, and power supplies to be of practical use.
In most microelectromechanical systems, each high voltage MEMS driver channel has a high impedance (to keep currents low) resistor divider creating a low voltage tap point proportional to the high voltage output. The feedback tap point is in turn coupled to a precision amplifier, normally auto-zeroed or nulled, which in turn drives a transconductor or level shift in a closed loop to create an output on the driver channel in conformance with an analog voltage or digital to analog (D/A) commanded input. The command input could be from a digital interface, a voltage level, a pulse-width modulation (PWM) input, or a similar input known to those skilled in the art.
Unfortunately, high valued resistors of high precision and with tight drift and temperature specifications and high initial accuracy are very expensive and difficult to source. Furthermore, arraying large numbers of high valued resistors across a printed circuit board creates high impedance nodes which are sensitive to noise coupling and leakage and uses a lot of space. Often these noise sources are introduced by the switching power supplies, usually boost converters, residing on or near the MEMS driver circuitry. The result is that most MEMS drivers have poor accuracy, “glitching problems,” poor power characteristics, and are often much larger than the MEMS devices they are trying to drive.
High voltage integrated circuits capable of coupling current onto a high voltage output with multiple channels are available. The high voltage integrated circuits accept either an analog input or a digital input and can command an output. In general high voltage integrated circuits either use internal resistors, in which case their relative accuracy is poor, especially over the drift of the process, or high voltage integrated circuits rely on an external resistor divider for each channel. Even if multiple on chip resistors are trimmed for good initial accuracy, a costly process, their drift and voltage coefficient specification is still relatively poor. Exotic materials like SiCr may be used, however, on-chip thermal drift between the different channels, chip topology and the drift of the material itself still do not meet the very tight accuracy requirements of many MEMS drivers or make the cost of such drivers prohibitive.
MEMs system which require multiple high voltage driver outputs also require a means by which to create the rails to drive those outputs. The power supplies required to generate high voltage rails from a low voltage battery can often be extremely large often requiring more than one stage due to the small duty cycle that would otherwise be required. Finally, most implementations of these power supplies are not monolithic and take up a great deal of space. The high voltage driver outputs pull their current from the highest voltage possible and therefore even if the required output voltage is small the power consumed is not reduced.
It would therefore be desirable to create a multichannel MEMS driver by which multiple channels in an integrated circuit may be trimmed against a single external precision low-drift resistor divider. Additionally, it would be desirable to incorporate the high-voltage rail generation control and switching circuitry on the integrated circuit to minimize electromagnetic interference (EMI) and printed circuit board (PCB) loops which can radiate and to save space. Finally, it would be desirable to be able to select rails for multiple outputs on a dynamic basis in conformance with the output voltage command so as to reduce the overall system power use (where rails lower than a given rail are grouped, and the rail sub-regulated to each channel). And finally, it would be desirable to monolithically include all of these components to save space and to utilize a dielectrically isolated process to isolate the power converter switching noise from the rest of the circuitry (ie. charge pumps and inductive boost converters).